Chemiresistive materials are a class of material that change their electrical resistance in response to an interaction with a chemical. In particular, chemiresistive materials commonly consist of a mixture of conductive particular matter coated or suspended in an essentially non-conductive material. The ratio of conductive to non-conductive material is such that the materials conductance or resistance can be measured using standard techniques. Generally the non-conductive material consists of, but is not limited to, an organic material such as a polymer or a self-assembled monolayer. It is generally accepted by those skilled in the art that these chemiresistive materials function through adsorption of chemical species into the organic material. This adsorption causes a swelling of the organic material, which subsequently increases the distance between the conductive particles, thereby causing an increase in the chemiresistive materials resistance.
Chemiresistors and arrays of chemiresistors for determining levels of analytes in the gas or liquid-phase are well-established in the art by measuring resistance changes of chemiresistive materials in the presence of particular analytes. One method of realising such a chemiresistor is to prepare thin films of gold nanoparticles that are coated with organic molecules (the chemiresistive material), and with two electrodes on either end. Wohltjen and Snow (Anal. Chem., 1998, 70, 2856) teach that the exposure of thin films of gold nanoparticle-based materials to organic vapours such as toluene in nitrogen carrier gas result in reversible film swelling. The swelling causes the conducting particles to move further apart which leads to an increase in the resistance of the chemiresistor.
A variety of materials can also be used as the sensing element for chemiresistors. For instance, carbon black can be mixed with a conducting or non-conducting polymer (Lonergan et al., Chem Mater., 1996, 8, 2298; Doleman et al, Anal. Chem., 1998, 70, 4177; Sotzing et at, Chem Mater., 2000, 12, 593) and deposited between two electrodes to form a chemiresistor for detecting gases or vapour. Moreover, graphene (Schedin et al., Nat. Mater., 2007, 6, 652) and carbon nanotubes (Wang et al., J. Am. Chem. Soc., 2008, 46, 8394) have attracted interest in recent years as materials that can change their conductivity in the presence of a chemical species. Such chemiresistors have been exclusively used for gas or vapour-phase detection of analytes.
WO 2008/092210 A1 to Raguse et al, teaches the use of chemiresistors in an electrolyte solution. To realise such a chemiresistor, the electrodes and chemiresistive materials are designed so that the chemiresistor film impedance is lower than the impedance due to the double layer capacitance of the total electrode surface in contact with the electrolyte solution. When exposed to toluene, dichloromethane or ethanol dissolved in the electrolyte solution, the nanoparticle film increases in resistance.
Raguse et al. (WO 2008/092210 A1 and J. Phys Chem C, 2009, 113(34), 15390) further teaches that the degree of interaction between an organic molecule dissolved in aqueous solution and the nanoparticle film is proportional to the partition coefficient between the two, and for hexanethiol-functionalized gold nanoparticles, mirrors the well-known octanol-water partition coefficient. However, these results indicate that ionically charged molecules, which have small water-octanol partition coefficients (i.e. which have a relatively high water solubility compared to non-polar, uncharged organic molecules) will partition into the chemiresistor film only poorly. Such weak interactions between ionically charged molecules and the chemiresistor materials would lead to only small changes in the chemiresistor resistance in the presence of such charged molecules. Thus there exists a need to improve the ability of chemiresistor sensors that function in electrolyte solutions to interact with ionic analytes in order to improve sensitivity and selectivity towards said charged analytes.
There are a number of applications where it would be advantageous to increase the interaction between chemiresistor materials and charged molecules dissolved in electrolyte solution. For instance, a large number of analytes of interest to the pharmaceutical, environmental or biomedical industries are charged molecules. These include, but are not limited to, various drugs, pesticides, herbicides, amino acids, peptides, metabolites.
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